Students often make mistakes with thousands, millions and billions. It might seem strange, but you should say "10 million" not "10 millions". It's the same with hundred, thousand and billion. Try to avoid this mistake in writing task 1 - examiners notice it!
Correct: 10 million people
Wrong: 10 millions people, 10 millions of people, 10 million of people
When there is no number, we do write "millions of".
e.g. Millions of people travel abroad each year.